This invention relates generally to a multi-layer magnetic random access memory (MRAM) device using spin-torque magnetic tunnel junction (MTJ) devices and a method to write state into a subset of the MTJ devices. In particular, stacked MTJ devices within the multi-layer MRAM device are written selectively using a combination of a hard axis magnetic field (i.e., hard magnetization) and spin injection.
Conventionally, an MTJ device is used as a memory element in a MRAM device. The MTJ device is a magnetoresistive device having a resistance that can be changed by an applied magnetic field. A binary data ‘1’ or ‘0’ is stored in the MTJ device by changing the resistance thereof to “high” or “low”, respectively. The resistivity of the MTJ device is determined by the direction of the magnetic field in the free layer of the MTJ device.
One problem with commercially available MRAM technology is that the size of the memory cell cannot be reduced without increasing the current used to write the MTJ device. Recently, a write method using a spin injection magnetization reversal process has been employed that may help to overcome some of the noted scaling issues related to the write current. Spin-polarized electrons (i.e., spin-injection currents) are injected directly into the magnetic recording layer of the MTJ device to reverse the magnetization of the magnetic recording layer. The spin-injection currents, however, still need to be reduced in order to achieve a compact memory cell due to the fact that the write current passes directly through the MTJ device and its associated selection transistor. Hence, the associated selection transistor may limit the memory cell size.
FIG. 1 is a circuit diagram illustrating a conventional spin-torque MRAM device. As shown in FIG. 1, a memory cell array 20 includes a single MTJ device which is connected to an upper bit line BLu, and a lower bit line BLd via a transistor Tr. As shown in FIG. 1, during a write operation of the MRAM, a word line WL is set at high by the word line driver WD and the transistor Tr is turned on. When a binary ‘1’ is written in the MTJ device, control signals A and B are set to low “L” and the control signals C and D are set to high. Since the transistors P1 and N2 turn on, a spin-injection current Is flows through the MTJ device in a direction from a driver/sinker 25 to a driver/sinker 30. In addition, the control signal E is set to low, and the control signal F is set to high, and the assist current Ia is passed through a write assistance line AL from a driver 35 to a sinker 40. The assist current Ia generates an assist magnetic field in the direction of the hard magnetization of the MTJ device. When “0” is written into the MTJ device, control signals A and B are set to high, control signals C and D are set to low. The transistors P2 and N1 turn on and the spin-injection current Is flows through the MTJ in a direction from the driver/sinker 30 to the driver/sinker 25. In addition, the control signal E is set to low and the control signal F is set to high and the assist current Ia directed from the driver 35 toward the sinker 40 is passed through the write assist line AL. Again the assist current Ia generates the assist magnetic field in the direction of the hard magnetization of the MTJ device. This conventional spin-injection magnetization reversal method uses the assist magnetic field in the direction of the hard magnetization to lower the potential barrier of the MTJ device so that it can be written with less spin-injection current.
It would be desirable to stack layers of these conventional MTJ devices one upon another to form a more compact memory cell overall. Unfortunately though, any applied magnetic field used to assist the in the process of writing a stacked MTJ MRAM device may impact MTJ devices on neighboring layers. Therefore, it is also desirable to “zero-out” an impact of the hard axis magnetic field on unselected MTJ devices occupying proximate layers above and below the selected memory cell.